Among the first things that anybody who ventures into IISc will notice is (apart from, e.g., how quaint the place is – seriously, the look of the place hasn’t changed since CV Raman) that every road at IISc seems to be coated in bird crap. I say ‘seems’ with reason:

a) If you are indeed venturing into IISc, you’re likely to take the roads most frequented by people. The bird-crap is particularly dense on these roads (go figure). There are other roads that are less densely crapped-on. (b) Also, bird-crap isn’t really ‘crap’. It’s the equivalent of urine in mammals. Birds don’t have separate digestive and excretory canals. There’s just the one outlet – the cloaca (this being also their reproductive orifice – even in male birds).

Avani ‘Chotey’ Mital and I wanted to see if we could make sense of this re-coating of already-metalled IISc roads. The short zoologist told me that birds are uricotelic, that uric acid (or urate, really) is poisonous, and that therefore, birds need to get rid of this shit as soon as their bodies make it.

That seems like a good explanation, doesn’t it? Turns out Chotey Mital is a good confabulator. Here’s why:

Birds are uricotelic. Uricotely, because it involves excretion in the form of concentrated spheres of urate, is the least water-intensive of the three possible types of excretory systems – uricotely, ureotely and ammonotely. Birds, aquatic birds for instance, which have enough access to water are found to be ammonotelic. Uric acid is also the least poisonous of the three.

There have also been studies that claim that birds are conditionally – facultatively – ammonotelic. A higher water intake will make them ammonotelic. So will lower temperatures with high food intake. So also, apparently,will lower nitrogen intake. We’ll see why. More recent studies have shown that this capacity for facultative ammonotely may be restricted to only a few species of birds.

The present paper is from the Journal of Experimental Biology and deals with two birds – the yellow-vented bulbul and Tristram’s grackle. In the study, several birds from each species were fed a variety of diets so that the study could control for sugar intake, nitrogen intake, and water intake, and monitor both the ureteral urine composition and the composition of the excreted urine. I am going to skip most of the methodology and even the numbers in the results, not least because I have no intuition for either of these, and go straight to what the results say.

First, birds are either uricotelic or ammonotelic, never ureotelic; the amount of urea in bird excreta is negligible. In the bulbuls, the study finds a positive correlation between ammonotely and water intake, and a negative correlation between protein intake and ammonotely. In the grackles, however, the composition of the excreta did not depend on water intake or on nitrogen intake. The experiments were carried out for three days, the authors say. They show that the study has not affected the physiology of the birds by showing that the amount of ammonia in urine shows no dependence on what day it is.

Futhermore, because the authors collected ureteral urine samples, they have been able to show that there is post-renal urine modification. This is the reason birds are apparently ammonotelic and not ‘truly’ ammonotelic. The uric acid in the urine (which is in the form of spherical concentrations) is dissolved and modified in the lower intestine. So although the amount of ammonia in the urine remains relatively unchanged, the concentration of ammonia would have gone up, sometimes past 50%, which makes the bird ammonotelic.

That something is done to the urine in the lower intestine is clear. The paper proposes two mechanisms for this hitherto unexplained process. The first one is that there are uricolytic microbes in the caeca of the birds, which would break down uric acid into ammonia.

But bulbuls have only vestigial caeca. That can’t be the explanation, at least in this case. The other explanation is that there is some uric acid transporter in the intestine of the birds. Uric acid is an antioxidant, and bulbuls belong to a lineage of birds that lacks the ability to synthesise this compound in the body. Reabsorption could be advantageous to the bird.

This capacity of facultative ammonotely, however, has not been seen in non-frugivorous birds. And crows are scavengers, not frugivores. Crows, then, are uricotelic. Which still leaves the question unanswered: why are IISc’s roads coated white?

Well, it turns out the explanation is much simpler: birds that don’t crap can’t take off and fly!